The Big Bang co-star’s relationship early in the CBS sitcom’s 12-season run was a big secret on set, with just a few colleagues knowing about their two-year romance at the time. 

“All the scenes up until we actually got together in real life, it was obvious there was always something there,” Cuoco recalled of their romance, as quoted in Jessicas Radloff’s oral history of the show. “There was chemistry, and we were crushing on each other. That was the whole first season until we actually got together for real.”

At Comic-Con, they’d pretend to say goodnight in the hotel lobby with a friendly hug, and then, Cuoco said, “we were like little mice in the night, running back and forth to each other’s rooms, trying not to be noticed. I mean, who did we think we were?”

Even though their real-life romance eventually burned out, Penny and Leonard’s chemistry wasn’t affected in the slightest, thanks to Cuoco and Galecki’s enduring affection for one another

“Johnny is actually one of my best friends,” Cuoco said on Watch What Happens Live in 2015. “It’s a situation that ended well. I understand sometimes it doesn’t. It could have gone horrible.”